1. Field of the Invention
This invention relates to laminate films useful in packaging in general and as encapsulants in photovoltaic modules in particular. The invention particularly relates to transparent packaging films comprising ethylene acid copolymer ionomers.
2. Description of the Related Art
Good optical properties are important in packaging materials in general and solar cell modules in particular because good performance requires that light incident to the cell be transmitted efficiently and effectively to the voltage-generating layer. Poor light transmission reduces the efficiency and/or productivity of the photovoltaic generation process.
For example, a common form of solar cell module is made by interconnecting individually formed and separate solar cells made for example of crystalline silicon, and then mechanically supporting and protecting the cells against environmental degradation by integrating the cells into a laminated solar cell module. The laminated modules usually comprise a stiff transparent protective front panel or sheet, and a rear panel or sheet typically called a “backskin”. Disposed between the front and back sheets so as to form a sandwich arrangement are the interconnected solar cells and an encapsulant.
A necessary requirement of the encapsulant (or at least that portion thereof that extends between the front sides of the cells and the transparent front panel) is that it be transparent to solar radiation. The typical mode of forming the laminated module is to assemble a sandwich comprising in order: a transparent panel, e.g., a front panel made of glass or a transparent polymer, a front layer of at least one sheet of encapsulant, an array of solar cells interconnected by electrical conductors (with the front sides of the cells facing the transparent panel), a back layer of at least one sheet of encapsulant, and a backskin or back panel, and then bonding those components together under heat and pressure using a vacuum-type laminator. The back layer of encapsulant may be transparent or any other color, and prior art modules have been formed using a backskin consisting of a thermoplastic or thermosetting polymer, glass or some other material.
A further requirement of the encapsulant is dimensional stability. In order to avoid potentially damaging stresses on the silicon cell, the encapsulant and surrounding structure should be stable to the temperature fluctuations that are found in end-use locations of the module.
A large number of materials have been used or considered for use as the encapsulant in modules made up of individual silicon solar cells. Ethylene vinyl acetate copolymer (commonly known as “EVA”) is commonly used as an encapsulant for modules comprising crystalline silicon solar cells. However, EVA may have certain limitations or deficiencies, such as its tendency to discolor. Also, it can decompose and release acetic acid. EVA also can require cross-linking—for example as described in U.S. Pat. No. 6,093,757—to impart dimensional stability. Cross-linking is a potential source of variability in the product, and can promote subsequent oxidation and degradation of EVA. In addition, EVA must be laminated in a vacuum when making a module because of the presence of peroxide as a cross-linking promoter in the EVA. EVA used as an encapsulant material usually contains 33% (by weight) of vinyl acetate, and thus is a very soft and tacky material that makes handling EVA in a manufacturing environment somewhat troublesome.
The use of ionomer as an encapsulant is described in U.S. Pat. No. 5,478,402, hereby incorporated herein in its entirety by reference. The use of ionomer as an encapsulant is further disclosed in U.S. Pat. No. 5,741,370. The term “ionomer” and the type of resins identified thereby are well known in the art, as evidenced by Richard W. Rees, “Ionic Bonding In Thermoplastic Resins”, DuPont Innovation, 1971, 2(2), pp. 1-4, and Richard W. Rees, “Physical Properties And Structural Features Of Surlyn® Ionomer Resins”, Polyelectrolytes, 1976, C, 177-197. Ionomers may be formed by partial neutralization of ethylene-methacrylic acid copolymers or ethylene-acrylic acid copolymers with organic bases having cations of elements from Groups I, II, or III of the Periodic Table, notably, sodium, zinc, aluminum, lithium, magnesium and barium. Surlyn® ionomers have been identified as copolymers of ethylene and methacrylic or acrylic acid that typically have a melting point in the range of 83-95° C.
It can be desirable to provide materials that are useful as encapsulant materials in photovoltaic cells, wherein cross-linking is not required for acceptable dimensional stability of the encapsulant material.